(a) Field of the Invention
This invention relates to a modified form of alumina hydrate crystals, methods of producing such crystals and thermosetting and thermoplastic compositions containing these crystals which impart improved workability to said compositions and improved bending strength properties to cured products obtained therefrom.
(b) Prior Art
It is known to those skilled in the art that resinous materials such as, for example, unsaturated polyesters, urethanes, vinyls, etc., burn rapidly at comparatively low temperatures. In view of these properties, it is necessary to incorporate one or more flame retardant materials in resinous compositions designed for uses such as in clothing, laminates used in the home, etc. These flame retardants added to resinous materials cause a reduction in the burning ability of the ultimately cured resinous or plastic products made therefrom.
One of the most common flame retardant additives used is alumina trihydrate (Al.sub.2 O.sub.3 3H.sub.2 O). This material has found wide application as a filler material in the plastics industry. Its principal applications are in such areas as rubber carpet backings, adhesives, glass-reinforced unsaturated polyester sanitary ware, appliance housings, molded electrical products, vanities, cross linked polyethylene insulation and cross linked acrylic wall panels. More detailed descriptions of the flame retardant characteristics of alumina hydrate, its properties and its application in the plastics industry can be found in the literature such as, for example, Woycheshin et al., "Handbook Fillers Reinforced Plastics", 1978, Chapter 14, pp. 237-249. Aluminum trihydrate appears to function as a flame retardant because the contained water molecules are liberated into the resin during burning. The heat of burning drives off the water vapor present and does so at a temperature that the liberated water absorbs heat generated during the burning, thereby reducing the rate of burning somewhat akin to quenching the flame.
Unfortunately, while alumina trihydrate is an excellent flame retardant, it is well known that its inclusion is resinous compositions results in a deterioration of some of the mechanical and/or chemical porperties of plastic compositions, particularly strength properties such as bending strength of the cured product and the workability (viscosity) of the uncured composition. The alumina trihydrate, heretofore used as a flame retardant, is commonly derived from the well-known Bayer process and is characterized by having a mean crystal diameter in the range of 40 to about 100 microns, with the individual crystals being sharp edged, strongly structured and fissured. It is also known in the art to use as a flame retardant Bayer process alumina trihydrate which has been ground to reduce the diameter of the crystals to a mean crystal diameter of about 25 microns or less. This grinding operation results in the alumina trihydrate crystals having substantially sharp corners and in a substantial amount of the alumina hydrate crystals being broken up into very fine fragments and a dust-fine abrasion, thereby producing ground alumina trihydrate wherein at least 1% by volume of the crystals have a diameter of less than 2 microns. These ground alumina trihydrate crystals, when incorporated into a resinous composition in substantial quantities desirable for achieving good flame retardant properties, cause a sharp rise in the viscosity of the resinous composition thereby making further processing of the composition impractical. Furthermore, it is also known that when sufficient quantity of these prior art ground alumina trihydrate crystals are incorporated into resinous compositions in order to achieve the desired flame retardant properties, curing of such compositions results in a substantial reduction in the bending strength properties of the cured product in comparison to a resinous product from which these ground alumina trihydrate crystals are absent. A similar deterioration of the bending strength properties of the cured resinous product results when coarser alumina trihydrate crystals having a mean crystal diameter of 40 to 100 microns are used.